A differential switch is commonly implemented using a differential pair of transistors, which have their emitters coupled together to a current source. The differential input to the switch is provided to the bases of the transistor pair, and the differential output is taken at the corresponding collectors. When the differential switch is enabled (current source is enabled), the differential pair inverts the signal applied to the bases, such that a positive going base signal causes a negative going collector signal (and vice versa, where a negative going base signal causes a positive going collector signal). Further note that, when the differential switch is enabled, both transistors of the differential pair will have current flowing through them continuously (assuming linear operation). The ratio of transistor currents (e.g., transistors Q1 and Q2, which make up the differential pair) will generally vary as a function of the applied differential input signal. When the differential switch is disabled (current source is disabled), both transistors of the differential pair are off (not conducting).
The current source is typically implemented with a third transistor, which has a constant bias voltage applied to its base. In operation, the differential switch is effectively controlled by the sign of the differential control signal applied to the bases of the differential pair. In more detail, energy from the current source passes through one transistor of the differential pair (e.g., the transistor having the higher base potential), and the corresponding output signal is provided at the collector of that transistor. The complement of this output signal is the output signal provided at the collector of the other transistor of the differential pair (e.g., the transistor having the lower base potential), thereby providing a differential output.
There are a number of ways to implement a conventional differential switch, whether by simple series switching or series-shunt switching techniques. In any such cases, a common problem associated with the switch is leakage. In particular, signal leakage though the series switching device degrades off-state isolation (i.e., when the switch is disabled, it still passes the output signal being applied at the inputs), especially at high frequencies.
There is a need, therefore, for differential switching devices having better leakage performance.